1. Field of the Invention
This invention relates generally to semiconductor manufacture. More particularly, the invention pertains to an improved semiconductor card and to improved methods and systems for fabricating the card.
2. State of the Art
One type of electronic assembly containing semiconductor components is generally referred to as a xe2x80x9ccard.xe2x80x9d Examples of such xe2x80x9ccardsxe2x80x9d include multimedia cards (MMC) such as used in digital cameras and the like, memory cards, smart cards, and personal computer memory card international association (PCMCIA) cards. In the industry, these types of cards are sometimes referred to collectively as xe2x80x9csemiconductor cards,xe2x80x9d xe2x80x9cmultimedia cardsxe2x80x9d or xe2x80x9cdaughter boards.xe2x80x9d
Typically, a semiconductor card is formed from a substrate, which may be a thin, printed circuit board (PCB) upon which electronic components are mounted. Such components may include, for example, at least one semiconductor die and/or die package as well as resistors, capacitors, inductors and the like to form a desired circuitry. The substrate includes conductors for providing power supply and interconnection of the various components. Typically, the components are mounted on one side, i.e., the xe2x80x9ccircuitxe2x80x9d side of the substrate, and are electrically interconnected to external contacts on the opposing side by interlevel conductors. The external contacts are arranged for electrical contact with a next level package, i.e., mother board. In use in an exemplary electronic apparatus, such as a digital camera, the card may be inserted into a slot or other receiver for interconnection with a motherboard and to provide, for example, flash memory for digitally recorded images.
Semiconductor cards are typically intended for repeated handling by the public, necessitating protection of the components from mechanical forces, moisture, radiation and stray electrical currents, etc. In addition, the surface materials of the card must be resistant to normal wear and tear. This is particularly true in peripheral portions of the card. In the industry, the semiconductor components and interconnecting conductors on the circuit side of a card substrate have typically been encapsulated by first applying xe2x80x9cglob topxe2x80x9d encapsulant. Then, a separately formed protective cover produced by injection molding is adhesively attached over the circuit side of the substrate to form the semiconductor card. However, use of a separately formed cover not only adds undesirable thickness to the card but requires additional process steps and is subject to deleterious detachment of the cover from the substrate. In addition, any variation in mounted component height and overlying glob top material will result in card thickness variation.
For most applications, it is desirable to make the card as thin as possible. The use of thin cards saves space within the equipment in which the card is used, as well as storage spaces, and savings in encapsulation material is also realized.
A further requirement for semiconductor cards is that the peripheral outlines and card dimensions be as smooth and uniform as possible, so that proper effective insertion into a card receiver is assured. The edges should not have any burrs. Specifications on the peripheral outline and dimensions of semiconductor cards have been set by various industry standard setting bodies, e.g., PCMCIA.
In present methods of manufacture, components for several semiconductor cards are fabricated and wire bonded on a strip of, e.g., circuit board. The strip may be viewed as equivalent to the lead frame in die manufacture. The individual cards are then separated from the strip using a singulation process such as sawing. Often the singulation step produces slivers or burrs and may form card edges which are rough or sharp. These defects can adversely affect the peripheral outline, dimensions, appearance and use of the card.
The need exists for a semiconductor card having improved uniformity in peripheral outline, dimensions, appearance, edge smoothness and performance. A method for producing an improved semiconductor card with reduced expenditure of time and materials is needed. In addition, the desired method will produce a card with improved precision in peripheral outline, dimensions and appearance, and at lower cost.
In accordance with the present invention, an improved semiconductor card is provided. The method of fabricating the improved card uses two molding steps and eliminates the need for glob top, cap molding and cap attachment. The method is rapid and uses molding processes well-known in the industry. In addition, the use of two molding steps makes available a variety of options not otherwise available. In addition, the method effectively eliminates resin bleed into areas intended to be left uncovered. The fabrication method provides cards with an enhanced degree of dimensional uniformity and reliability.
The semiconductor card includes a substrate such as a printed circuit board (PCB). The substrate comprises an electrically insulative material such as an organic polymer resin reinforced with glass fibers and may include more than one layer.
A substrate may be initially formed as part of a substrate strip containing more than one substrate-frame module. Each substrate is defined by a peripheral opening in the module. The strip is similar in function to a semiconductor lead frame and permits various fabrication processes to be performed on one or more substrates at the same time. The substrate is connected to the strip with connecting segments similar to tie bars on a semiconductor lead frame. The substrate has a circuit side with a pattern of conductors thereon and an opposing back side with a pattern of external contacts thereon. Electronic components such as semiconductor dice, resistors, capacitors, and the like are formed or mounted on the circuit side of the substrate. The semiconductor dice may comprise, for example, bare dice wire bonded to the conductors, bumped dice flip chip mounted to the conductors, or semiconductor packages bonded to the conductors.
In two molding steps, a first plastic body is first formed on the circuit side of the substrate to encapsulate the circuit components including semiconductor components; a second plastic body is then molded to complete the desired card geometry. Typically, a xe2x80x9ctransfer moldingxe2x80x9d apparatus is used for each molding step, but the cavity configuration for each step is different.
The first molding step is similar to well-known methods for chip-on-board (COB) molding wherein semiconductor components are encapsulated to a circuit board. In the first molding step, peripheral portions of the circuit side of the substrate are left uncovered, to be covered by a second plastic body in a second molding step.
In the second molding step, the second plastic body is formed on the peripheral portions of the circuit side and extending laterally therefrom to cover the substrate edge. The second body may also extend over portions of the back side of the substrate. To prevent bleeding of mold resin to the back side of the substrate, a major portion or all of the substrate back side is forced against a mold plate by pressure of the opposite mold plate on the first plastic body.
The first and second molding steps may use different resins to maximize both circuit protection and card life.
The use of two molding operations enables a wide variety of optional card configurations. Several representative embodiments of the second molding step are described herein, each using a particular configuration of mold plates.
Following the second molding step, the card is cut from the surrounding frame. Singulation of the card from the frame or strip results in a card having a smooth, burr-free peripheral edge which has a rounded, oblique or other desirable edge shape. Exposed ends of connecting segments may be recessed into notches formed into the card edges.